The present invention relates generally to circuitry for detecting current overloads from a voltage source to a load. In particular, the invention relates to a technique for allowing the precise setting of a trip current value by means of a resistance network.
In industrial settings, induction motors are often used to drive electric machinery. These motors are generally designed to operate within a certain range of current. Exceeding this current range can lead to excessive heat generation which can damage the motors. Overload relays are frequently utilized to prevent this damage from occurring. In these settings, overload relays measure the current entering the motor from the power supply. If a current overload is detected, the overload relay disconnects the motor from the power supply. The trip current for these relays is generally set by manually adjusting the resistance of an internal current overload detection circuit.
Frequently, such adjustment is facilitated by a potentiometer within the current overload detection circuit. Most often, a rotatable dial is used to set the trip current for the overload relay by varying the resistance of the potentiometer. The current and resistance of any circuit are inversely related. Thus, as the resistance of the potentiometer increases, the current decreases; and as the resistance of the potentiometer decreases, the current increases. However, the current and resistance of the circuit are reciprocally related, not linearly. As a result, while the resistance of the potentiometer changes linearly with the rotation of the dial, the corresponding trip current changes in a non-linear fashion. This relationship is governed by Ohm's Law, which states that the voltage across a resistor is equal to the current through the resistor multiplied by its resistance. Thus, in the case of a desired 10V burden voltage, trip current settings 1A, 2A, 3A, 4A, and 5A would require resistances of 10Ω, 5Ω, 3.3Ω, 2.5Ω, and 2Ω respectively. Thus, if a 10Ω potentiometer is used, a setting of 1A occurs at one end of the potentiometer's range (when set to 10Ω), but 2A operation occurs in the middle of the potentiometer's range at 5Ω. As a result, the higher the trip current needs to be set, the closer the markings must be to each other, leading to a crowding of the higher settings toward the opposite end of the potentiometer. Consequently, the dial angle between these higher current settings is quite small, which presents difficulties in precisely setting the trip current and increases the possibility of operator error damaging to the motor. In addition, the uneven spacing between settings is often simply inconvenient and non-intuitive for users.
In an attempt to overcome this crowding problem, custom potentiometers have been developed. These custom potentiometers are made so that the resistance varies non-linearly with respect to the rotation of the dial. A standard potentiometer is usually formed from one resistor and a sliding mechanism. The resistance of the resistor is consistent throughout, so that the resistance changes linearly according to the length of the resistive element from the point at which current is input to the point at which the slider makes contact with the resistor. Custom potentiometers incorporate a similar sliding mechanism, but the resistor utilized is non-linear. Such a custom potentiometer may be constructed of two resistors with different resistive properties; may incorporate a great number of resistive elements, each with different resistive characteristics; or may utilize a single resistive element with resistive properties that vary through the length of the element.
These custom potentiometers may reduce the problem of the crowding of higher current settings toward one end of a potentiometer, but there are shortcomings associated with the incorporation of such a custom potentiometer within an overload relay. One such drawback is the expense involved in using custom potentiometers. Custom potentiometers are much more difficult to manufacture than standard, linear potentiometers. The resistances have to be carefully selected and spliced together. As a result, they are more expensive than linear potentiometers. Additionally, these custom potentiometers are only partially effective in addressing the issue of uneven marking and crowding of settings. The simpler versions of custom potentiometers incorporate two different resistors spliced together. This only reduces the problem. As the slider moves across the resistors, the trip current being set still changes in a non-linear fashion. The current settings of the dial still experience some level of crowding, but the crowding is less severe and occurs at both one end of the potentiometer range and near the middle of its range as the slider nears the end of the first resistor. An even more expensive, three-resistance custom potentiometer would experience somewhat less crowding than the two-resistance version, but the crowding would occur at three locations of the potentiometer's range instead of two.
Clearly, there is a need for an overload relay with dial markings that are evenly spaced about the dial. Such an arrangement would allow for more precise setting of a trip current for an overload relay, reducing the risk that human error would result in expensive damage to complex machinery. There is a particular need for an overload relay that can fulfill these objectives through the use of a simple, linear potentiometer.